Diffusion pumps are standard components in high vacua systems. The working principle of diffusion pumps is the use of a directed stream of molecules to impact on randomly moving air molecules. The impact transfers momentum to the air molecules and sweeps them along with the directed stream. If a diffusion pump is placed between a chamber to be evacuated and a positive displacement vacuum pump, then the residual air molecules can be swept into the entrance of the positive displacement pump and exhausted into the atmosphere.
Provision must be made for separation of the directed molecular streams if the molecules are to be re-used. In practice, a condensible fluid (originally mercury, but now more often hydrocarbon, silicone oil or polyphenyl ether) is boiled to produce the directed stream of molecules, and then condensed back to a liquid to separate it from the outgoing air molecules. The practical implementation of this diffusion pump is to use a vertical tube as the pump body which is closed at the bottom, and provided near the lower end with a side exit conduit for connection to a positive displacement pump.
A boiler and jet assembly is positioned centrally within the pump body and produces several annular jets (directed streams) traveling towards the lower end of the pump and impinging eventually on the inner surface of the pump body. The upper portion of the pump body is cooled so that the gas streams condense and run down the inner surface of the pump body and into the boiler of the boiler and jet assembly. An external heater re-heats the condensate to boiling and introduces the resultant vapor into the jet assembly to repeat the process. If the open upper end of the pump body is attached to a vessel to be evacuated, then air molecules entering the pump body are swept downwards and into the positive displacement pump.
This prior art pump design has several disadvantages. The upper wall of the prior art diffusion pump is cooled to provide condensation while the lower wall is heated to boil the fluid. As the wall must be strong enough to resist atmospheric pressure, it is relatively thick and a considerable amount of the heat energy goes directly from the boiler into the cooling medium (water or forced air). Since the cooling medium is usually water, and since the boiler is exposed to open air, the working ranges of the fluids are limited to those materials which boil at relatively low temperatures, and which are liquid at the temperature of cooling water.
This limitation on the working range of the fluids used in these prior art diffusion pumps limits the performance of the prior art diffusion pumps. Mercury has a vapor pressure of about 1.times.10E-3 mm Hg at room temperature. Although air will be pumped from the evacuated vessel by diffusion pumps using mercury vapor, it (air) will be replaced by mercury vapor which is not further condensed on the room temperature walls of the diffusion pump. The organic working fluids referred to above have much lower vapor pressures at room temperatures but the heating and boiling process "cracks" the molecular structure and results in the occurrence of volatile light hydrocarbons in the vacuum system. Most clean vacuum systems use a second condenser (water, freon or liquid nitrogen cooled) between the entrance of the diffusion pump and the vacuum chamber. These secondary condensers are usually in the form of cooled baffles. Such cool baffles are effective, but reduce the probability of air molecules entering the top of the diffusion pump, and thus reduce pumping speed. It is generally impractical to keep the baffles cold indefinitely so that at warm-up times hydrocarbon fractions or mercury is emitted into the vacuum vessel. Some materials, such as silicone oils form "creep" films which eventually migrate to the vacuum side of these cold baffles and hence into the vacuum systems.